EP0365884B1

EP0365884B1 - Corrosion resistant nickel-base alloy

Info

Publication number: EP0365884B1
Application number: EP89118438A
Authority: EP
Inventors: Edward Lee Hibner; Ralph Webb Ross, Jr.; James Roy Crum
Original assignee: Inco Alloys International Inc
Current assignee: Huntington Alloys Corp
Priority date: 1988-10-21
Filing date: 1989-10-04
Publication date: 1993-12-08
Anticipated expiration: 2009-10-04
Also published as: US5120614A; AU4360489A; AU611331B2; CA1334800C; JPH02156034A; DE68911266D1; EP0365884A1; DE68911266T2

Description

The subject invention is directed to a nickel-chromiummolybdenum-niobium alloy which affords a combination of exceptionally high resistance to various subversive corrosive media together with satisfactory weldability, stability, strength, etc.

INVENTION BACKGROUND

As is well known, nickel-chromium-molybdenum alloys are extensively used commercially by reason of their ability to resist the ravages occasioned by the aggressive attack of various corrosives, notably chlorides which cause crevice corrosion and oxidizing acids which promote intergranular corrosion. Alloys of this type are commonly used in the more severe corrosive environments and usually must be welded to provide desired articles of manufacture, e.g., tubing, large containers/vessels, etc. As such and in use, these articles are exposed to elevated temperatures and this gives rise to a problem of additional concern, to wit, corrosive attack at the weld and/or heat affected zone (HAZ). This problem is well known to, for example, the chemical process industry where more than passing attention is given to the gravity of attack.
To determine the likelihood of intergranular attack an ASTM test (G-28) is often used whereby an alloy is exposed to a temperature of circa 1400-1700°F (760-927°C) prior to exposure in given corrosives to ascertain its propensity to undergo attack. It is often referred to as a "sensitizing" temperature, i.e., a temperature deemed "sensitive" in predicting attack. There are two ASTM G-28 tests, the ASTM G-28 Method "B" test being deemed more reliable in determining this "sensitivity" as opposed to the ASTM G-28 Method "A" Test.

INVENTION SUMMARY

In any case, it has now been found that a nickel-base alloy containing correlated percentages of chromium, molybdenum, tungsten and niobium offers an excellent level of corrosion resistance as reflected by the standard ASTM G-28 Modified "B" Test. Moreover, provided the alloy chemistry is properly balanced, a good combination of alloy weldability, workability, strength, etc. obtains. Also of importance it has been determined that the alloy is most suitable for forming clad metal products, i.e., as cladding to steel. Furthermore, the structural stability of the alloy is excellent at low temperatures, thus rendering the alloy potentially suitable at cryogenic temperatures.
In addition to the foregoing, it has been found that the alloy is not adversely affected over a desired range of heat treatment temperature. In terms of an annealing treatment it has been found that temperatures of 2000°F (1093°C) and up at least to 2200°F (1204°C) can be utilized. This means that mill products, e.g., sheet, strip, plate, etc. can be made softer such they are more amenable to forming operations such as bending and the like. A temperature such as 2000°F (1093°C) is also beneficial in striving for optimum tensile strength.

INVENTION EMBODIMENTS

Generally speaking and in accordance herewith, the present invention contemplates a highly corrosion-resistant, nickel-base alloy as set out in the accompanying claims.
In terms of the alloying constituents chromium is important in conferring general corrosion resistance. Below 19% resistance drops off whereas much above 23% undesired morphological phases can form particularly at the higher molybdenum and niobium levels. A chromium range of 20 to 22.5% is deemed quite satisfactory. Molybdenum imparts resistance to pitting and is most beneficial in achieving desired critical crevice corrosion temperatures (CCT). Critical crevice temperature is important because it is a relatively reliable indicator as to the probability for an alloy to undergo crevice corrosion attack in chloride solutions, the higher the temperature the better. (A 6% FeCl₃ solution is often used for test purposes.) It is preferred that molybdenum be from 12.5 to 14.5%. Excessive molybdenum, say 16%, particularly with high chromium-niobium-tungsten levels, promotes instability through the formation of undesirable structural phases, e.g., Mu, whereas levels below, say, 12% detract from corrosion behavior.
Tungsten has a beneficial effect on weldability, enhances acid-chloride crevice-corrosion resistance and is considered to lend to imparting resistance to stress-corrosion cracking (SCC) of the type that occurs in deep sour gas wells (DSGW). It has also been noted that it increases the resistance to surface cracking due to carbon diffusion during heat treating to simulate cladding to steel. Tungsten levels of, say, 1.5-2% are inadequate and percentages above 4% are unnecessary. A range of 2.75 to 4% is advantageous.
Niobium enhances acid-chloride crevice corrosion resistance as will be shown in connection with the ASTM G-28, Modified "B" test and is deemed to offer greater resistance to SCC in deep sour gas wells. However, in amounts of 2% it tends to impair weldability and is detrimental to crevice-corrosion resistance in, for example, concentrated hydrofluoric acid. It should preferably be maintained below 1.5%, a range of 0.75 to 1.25% being satisfactory.
In terms of other constituents, titanium detracts from desired properties and preferably should not exceed 0.5%. Carbon advantageously should be maintained below 0.03% and preferably below 0.015 or 0.01%. Aluminum is beneficial for deoxidation and other purposes but it need not exceed 0.5%, a range of 0.05 to 0.3% being suitable. Silicon should be held to low levels, e.g., below 0.3%. The iron content is preferably from 3 to 6%.
The following information and data are given to afford those skilled in the art a better perspective as to the nature of the alloy above described.
In Table I below are given the compositions of the alloy of the present invention (Alloy 1) and an excellent commercial alloy (Alloy A). In respect of Alloy 1 a 30,000 pound (14000 kg) melt was prepared using vacuum induction melting followed by electroslag remelting. Alloy 1 was hot worked to 0.25 inch (0.64 cm) plate specimens which were then tested in various conditions as reported in Table II. In this connection "mill annealed" plate was cold rolled (CR) and/or heat treated to ascertain the effects of thermomechanical processing on corrosion resistance. Alloy A was tested as 0.25 inch (0.64 cm) plate.

Both ASTM G-28 Method "A" and Method "B" corrosion tests were employed. The Method "B" test, as indicated previously, is deemed more sensitive than "A", and more reliably identifies microstructures responsible for reduced intergranular corrosion and localized corrosion resistance.

TABLE I

Chemical Compositions*
Alloy	C	Mn	Fe	Si	Ni	Cr	Al	Ti	Co	Mo	Nb	W
1	.006	.23	4.60	.06	55.38	21.58	.15	.02	.48	13.62	.75	3.11
A	.004	.26	5.07	.06	55.96	21.31	.21	.02	.49	13.17	n.a.	3.02
n.a. - not added

*Alloys contained Mg and impurities

The data in Table II reflect that in respect of the more sensitive ASTM "B" test, Alloy 1 performed better than Alloy A. The effect of annealing temperature after cold rolling on resistance to subsequent sensitization at 1600°F (871°C) is shown in the first set of data. Test "B" shows that resistance to sensitization is founded by an anneal at 2050°F (1138°C) or higher for Alloy 1 and 2100°F (1149°C) anneal or higher for Alloy A. This difference in effective stabilizing anneals is considered to be a reflection of the 0.75% niobium in Alloy 1. The inability of Method A to detect sensitization of either alloy in this series of tests confirms that ASTM G-28 Method A is not as good a barometer for this type of alloy. It might be added that the ability to use a low annealing temperature (2050°F/1121°C versus 2100°F/1149°C) lends to higher strength.
The mill anneal temperature for Alloy 1 of the second group of data was 2100°F (1149°C) and 2050°F (1121°C) for Alloy A. Again, the Method A test was virtually insensitive in respect of either alloy over the 1400-2000°F (760-1093°C) sensitising temperature range whereas ASTM "B" resulted in severe sensitization at the 1600°F (871°C) temperature. Microstructures were examined, and heavy intergranular precipitation was observed.
Alloy 1 was further tested under a third processing condition as shown in Table II, i.e., mill anneal plus a 50% cold roll followed by 1700 to 2000°F (927-1093°C) anneals. Method "A" was again insensitive. In marked contrast, Test "B" resulted in considerable attack with the 1700 and 1800°F (927 and 982°C) anneals.

Apart from the above, critical crevice corrosion temperature data are given for Alloy 1 in Table III in a 10.8% FeCl₃ solution.

TABLE III

Alloy	Condition	Critical Crevice Temperature
1	mill anneal, 2100°F (1149°C)	55°C
1	m.a., CR 50% + 1800°F (982°C) /7 min., W.Q.	<45°C
1	m.a., CR 50% + 2000°F (1093°C) /7 min., W.Q.	55°C

The data in Table III reflect that an 1800°F (982°C) anneal is too low whereas the mill anneal (2100°F/1149°C) and 2000°F (1093°C) anneal gave excellent CCT results.

In Table V additional critical crevice corrosion temperature data are given for several alloys including Alloy A and the present invention, the chemical compositions being set forth in Table IV. A 6% Fe Cl₃solution was used for test and evaluation purposes. Alloys 2-5 are within the invention whereas A-G are outside the invention. Commercial Alloys 625 and C-276 are included for comparison purposes.

TABLE V

Alloy	Critical Crevice Temperature, °C
2	55.0; 55.0
3	55.0; 55.0
4	55.0; 55.0
5	55.0; 55.0
A	55.0; 55.0
B	42.5; 42.5
C	47.5; 47.5
D	47.5; 47.5
E	47.5; 47.5
F	50.0; 50.0
G	52.5; 52.5
Alloy 625	25.0 to 30.0
Alloy C-276	45.0 to 50

It will be observed that the alloys within the invention all had higher critical crevice corrosion temperatures than the alloys outside the invention save Alloy A. Alloys D and G contained marginally high niobium and Alloys such as B and D suffered from a deficiency of tungsten. Alloy F reflects that Ti is not a substitute for niobium.
With regard to weldability behavior alloys both within and without the invention (Table VI) were tested using gas metal arc welding (GMAW) procedures. This technique was used to evaluate HAZ microfissuring sensitivity because of its potency in producing this form of cracking as a consequence of its high heat input, shallow thermal gradients and high deposition rate. HAZ microfissuring is a problem particularly in respect of high alloy nickel-base alloys. It occurs as a result of macrosegregation and thermal gradients during welding.
One-half inch plates (Alloys 1, 2 and C) were prepared by annealing at 2100°F (1149°C)/1 hr. followed by air cooling. The edges of two 4-inch lengths of plate from each heat were beveled to 30 degrees for welding access. Two plates from each heat were prepared and welded down to a strong back for full restraint. The weld joint was produced using 0.035 inch (0.09 cm) diameter INCONEL® alloy 625 filler metal in the spray transfer mode. The welding parameters were 200 amps, a 550 inches/min. (14 m/min) wire speed, a voltage of 32.5 volts and 60 cfh (1.7 m³/h) argon as a shield. The weld faces were ground flush to the base metal, polished to 240 grit and liquid penetrant inspected for the presence of large microfissures.
Four transverse sections were taken from each heat. Three of the sections from each heat were machined, polished to 240 grit and bent at their HAZ's as 2T guided side bends. Alloy 2 did not show any indication of cracking (microfissures) whereas Alloy C depicted 8 HAZ cracks in the side bends. The remaining sections were mounted and polished for metallographic examination and optically examined for microfissures. Alloy 2 exhibited extensive HAZ grain boundary liquations with good back-filling to a length of 0.01 inch (0.3 mm) into the heat affected zone. No microfissures were observed. Alloy C showed poor back-filling (fissures), the liquation being 0.0175 (0.44 mm) inch into the HAZ. The grain size was approximately ASTM #4 in each case. It is considered that the carbon content of Alloy C, 0.021%, was high. In striving for best results the carbon content should not exceed 0.015% and preferably not more than 0.01%.
Alloy 1 was examined in the hot-rolled condition and also as follows: 1950°F (1066°C)/0.5 hr., WQ; 2100°F (1149°C)/0.5 hr., WQ; and 2150°F (1177°C)/0.5 hr., WQ. Parameters were: 0.061"dia. Alloy 625 filler metal, 270 amps, 190 in./min. (4.8 m/min) wire speed, 33 volts, 60 cfh (1.7 m³/h) argon and fully restrained. Weldments were ground, polished and liquid penetrant tested on the weld face and root. No cracking was noted. Radiographic examination did not reveal cracks. 2T side bends failed to exhibit any cracks. Two transverse metallographic sections were cut, mounted, polished and etched for each weldment and grain size conditions. Grain boundary liquation was from 0.0056 to 0.015 inch (0.14 to 0.38 mm) into the HAZ and the grain size varied from ASTM #6 to 1.5. No cracks, fissures or lack of back-fill were detected.
Data are tabulated in Tables VII and VIII. TABLE VII

Side Bend (2T) Results

Alloy Grain Size Bends Length of HAZ Grain Boundary Liquation, inch (mm)

2 4 Good 0.01 (0.25)

C 4 Poor 0.0175 (0.44)

TABLE VIII

Alloy Grain Size Cracks Length of HAZ Grain Boundary Liquation, inch (mm)

2 4 No 0.01 (0.25)

C 4 Yes 0.0175 (0.44)

1 1.5-6 No 0.015-0.0056 (0.38 - 0.14)

Gas metal-arc welding was used to examine Alloys B, E, D and G of Table VI. In this case 3/8 inch (1 cm) strip (3/8" x 2" (1x5 cm) length) was used for test purposes, the strip having been annealed at 2100°F (1149°C) for 1/2 hour. The 2T bend test was used, the parameters being: 0.062 inch dia. INCONEL filler metal 625; 270 amps; wire feed 230 in./min. (5.8 m/min), 32 volts and 50 cfh (1.4 m³/h) argon shield. Results are given in Table IX.

TABLE IX

Alloy	Grain Size, ASTM	Side Bend Weld Centered	Side Bend* HAZ Centered	Face Bend Weld Centered
B	4.5	No Cracks	No Cracks	Numerous Cracks at Fusion Line
D	4	No Cracks	No Cracks	Numerous Cracks at Fusion Line
E	5	No Cracks	No Cracks	Mini-cracks at Fusion Line
G	4	1,2 Cracks** Approx. 1/16" Long (0.16 cm)	1,2 Cracks** Approx. 1/16" Long (0.16 cm)	No Cracks

*2 tests per weld
**Cracks at fusion line running into HAZ

As indicated hereinafter, the alloy of the invention is particularly suited as a cladding material to steel. This is indicated by the data presented in Table X. A 2T bend sheet was used to study the effect of carbon diffusion from a carbon steel on Alloys B, D, E and G. While these particular compositions are outside the invention for other reasons, they nonetheless serve to indicate the expected behavior of alloys within the scope of the invention. The heat treatment employed with and without being wired to the carbon steel was adopted to simulate the steel cladding as shown in Table X. Included are data on commercial Alloy C-276.

TABLE X

Material Condition
Alloy	Heat Treated to Simulate Steel Cladding**
	As-Produced*	a. Not wired to C-Steel	b. Wired to C-Steel
B (1Nb,1W)	NC***	NC	3 cracks ****
D (2Nb,1W)	NC	NC	Multiple cracks****
E (1Nb,2W)	NC	NC	NC
G (2Nb,2W)	NC	NC	NC
C-276 (commercial sheet)	NC	NC	Multiple cracks****

* As-produced material = 1/8" (0.3 cm) strip in the 50% cold worked + 1149°C/15 min/AC condition.
** Heat treatment = 1121°C/30 min/AC + 593°C/60min/AC.
*** NC = No Cracking.
**** Where the specimen touched the steel during heat treatment. Note: For specimens heat treated wired to C-steel, the surface which contacted the steel was on the outside when bent.

Only the alloys containing nominally 2% tungsten were resistant to surface cracking related to carbon diffusion from the steel.

As indicated above herein, the subject alloy manifests the ability to absorb high levels of impact energy (structural stability) at low temperatures. This is reflected in the data given in Table XI which includes reported data for a commercial alloy corresponding to Alloy A.

TABLE XI

Alloy	Condition	Test Temp.,°F(°C)	Charpy V-Notch Impact Strength, ft-lbs(Nm)	Comments
1	Annealed 2100°F (1149°C)	72(22)	--	Did Not Break
1	Annealed 2100°F (1149°C)	-320(-196)	--	Did Not Break
1	Annealed 2100°F (1149°C) + 1000 hr. at 1000°F (538°C) AC	72(22)	>240(325)	Did Not Break
1	Annealed 2100°F (1149°C) + 1000 hr. at 1000°F (538°C) AC	-320(-196)	>240(325)	Did Not Break
A	Annealed 2050°F (1121°C) + 1000 hr. at 1000°F (538°C) AC	72(22)	259(351)	Did Not Break
A	Annealed 2050°F (1121°C) + 1000 hr. at 1000°F (538°C)	-320(-196)	87(118)	Broke

Representative mechanical properties are given in Tables XII, XIII and XIV, Alloy 1 being used for this purpose.

TABLE XII

Room Temperature Tensile Properties: Annealed Condition
Product	0.2% Y.S. ksi(MPa)	T.S. Ksi(MPa)	% Elong.	Hardness	ASTM Grain Size
0.650" (1.6 cm) Plate*	115.3(795)	150.0(1034)	32	Rc 31	--
0.650" (1.6cm) Plate	49.2(339)	104.6(721)	65	Rc 87	2
0.650" (1.6 cm) Plate	45.3(312)	102.5(707)	70	Rc 86	1-1/2

*As hot rolled

TABLE XIII

High Temperature Tensile Properties Annealed 0.250" (0.64 cm) Plate
Test Temperature °F (°C)	0.2% Y.S. ksi(MPa)	T.S. ksi(MPa)	% Elongation
200 (93)	41.1 (283)	98.7 (680)	67
400 (204)	35.2 (243)	91.7 (632)	70
600 (315)	31.7 (219)	87.5 (603)	69
800 (426)	29.8 (205)	85.0 (586)	68
1000 (538)	32.1 (221)	79.7 (549)	64
1200 (649)	27.6 (190)	77.0 (531)	62
1400 (760)	29.3(202)	69.0 (476)	53

TABLE XIV

Effect of Aging on Tensile Properties: 0.250" (0.64 cm) Annealed Plate
Condition	0.2% Y.S. ksi(MPa)	T.S. ksi(MPa)	% Elong.	Hardness
As Annealed	45.3 (312)	102.5 (707)	70	Rb 86
Anneal + 1000°F (538°C) 1000 Hr, AC	48.5 (334)	106.6 (735)	65	Rb 87

The presence of niobium in the weld deposits is considered to aid room temperature tensile strength as reflected in Table XV. Tests were made on a longitudinal section taken through the weld metal.
The subject alloy can be formed into a variety of mill products such as rounds, forging stock, pipe, tubing, plate, sheet, strip, wire, etc., and is useful in extremely aggressive environments as may be encountered in pollution-control equipment, waste incineration, chemical processing, processing of radioactive waste, etc. Flue Gas Desulfurization is a particular application (scrubbers) since it involves a severe acid-chloride environment. Oxidizing and cleansing elements may be present in small amounts. For example, magnesium or calcium can be used as a deoxidant. It need not exceed (retained) 0.2%. Elements such as sulphur and phosphorus should be held to as low percentages as possible, say 0.015% max. sulphur and 0.03% max. phosphorus. A practical commercial phosphorus range is about 0.005% to about 0.015%. While copper can be present, it is preferable that it not exceed 1%. The alloy range of one constituent of the alloy can be used with the alloy ranges of the other constituents.

Claims

A nickel-base alloy having
(a) a high degree of corrosion resistance to aggressive corrosive media, particularly in the heat affected zone when welded,

(b) good weldability,

(c) a high critical crevice corrosion temperature, and

(d) structural stability,
said alloy consisting, by weight, of 19 to 23% chromium, 12 to 15% molybdenum, 2.25 to 4% tungsten, 0.65 to less than 2% niobium, 2 to 8% iron, up to 0.1% carbon, up to less than 1% manganese, up to less than 0.5% silicon, up to 0.5% aluminum, up to 0.5% titanium, up to 0.2% total retained magnesium and/or calcium, up to 0.015% sulphur, up to 0.03% phosphorus and up to 1% copper, the balance being nickel plus incidental impurities.
An alloy according to claim 1, wherein one or more of the following elements is present in the following ranges:- chromium 20 to 22.5% molybdenum 12.5 to 14.5% tungsten 2.75 to 4% niobium 0.75 to 1.25% iron 3 to 6% carbon up to 0.015% manganese up to 0.5% silicon up to less than 0.3% aluminum and titanium up to 0.3% each.
An alloy according to claim 1 or claim 2 that contains 20 to 22.5% chromium, 12.5 to 14.5% molybdenum, 2.75 to 4% tungsten, 0.75 to 1.25% niobium, 3 to 6% iron, up to 0.015% carbon, up to 0.5% manganese, up to less than 0.3% silicon, up to 0.3% each aluminum and titanium.
An alloy according to any preceding claim in which the niobium content is from 0.75 to 1.25%.
A weld deposit structure in which the base metal is formed from an alloy according to any preceding claim.
A wrought composite metal clad product comprising a metal cladding bonded to a base metal, said cladding metal being formed from an alloy according to any one of claims 1 to 4 and wherein the base metal is selected from carbon steels, low and medium alloy steels.
Use of an alloy as claimed in any one of claims 1 to 4 for components that are exposed to aggressive environments, e.g. in pollution control equipment, waste incinerators, chemical process apparatus, apparatus for processing radioactive waste and gas desulphurization equipment.